Role of CD4+ regulatory T cells in hyperbaric oxygen-mediated immune nonresponsiveness.

We have previously shown that hyperbaric oxygen culture (HOC [95% O(2), 5% CO(2), 25 psi]) is an effective pretransplant tissue-modification technique that results in long-term allograft survival and the induction of systemic immune tolerance in a murine model. Here we address the immune modulatory effects of HOC-treatment of human immune responses using the in vitro mixed lymphocyte reaction (MLR). Pretreatment of allogeneic stimulator cells with HOC results in abrogation of cytotoxic T lymphocyte (CTL) activity, proliferative responses, and IFN gamma production in a 7-day MLR. These responses can be restored either by the addition of IFN gamma or IL-2 on day 0, or by blocking the activity of IL-4 and IL-10. The addition of IL-2 on day 4 does not restore allospecific CTL activity. The failure of HOC-treated cells to induce allospecific CTL is not due to the induction of anergy, demonstrated by the failure to restore responses after restimulation with allogeneic cells in the presence of IL-2. Removal of CD4(+) cells prior to restimulation, results in restoration of CTL activity in MLR cultures restimulated with HOC-treated allogeneic cells. These results suggest that HOC-induced immune nonresponsiveness is mediated by the development of CD4(+) regulatory cells in a Th2-type environment.